The present invention is generally directed to marking substrates with barcode indicia which are readily capable of being read by machine with a very low error rate especially when compared to optical character recognition systems. More particularly, the present invention is directed to a method for marking barcodes in a continuous fashion as a means for providing uniformity in line width and use on a variety of substrate materials. Even more particularly, the present invention is directed to the writing of single width barcodes on various materials using laser radiation so as to produce serpentine or boustrophedonic patterns which enhance both writing and reading barcode characteristics.
The present invention has grown out of work in the marking of semiconductor wafers with a single width barcode which is suitable for the materials and the processes involved in wafer processing and manufacture to produce electronic circuit chip devices. In particular, it has been found that writing multiple width barcode indicia on semiconductor materials using a laser is very difficult. Multiple width barcodes possesss solid lines which are both wide and narrow, and because of the nature of the semiconductor material, lines which are disposed adjacent to one another to produce or effectuate multiple width bar code patterns tend to be very difficult to read and correspondingly also very difficult to write in a fashion which renders them sufficiently well adapted to be read.
Accordingly, especially when writing on semiconductor materials and especially when employing laser energy to produce the markings, both of which are very desirable, it has been found that it is necessary to use single width barcodes as opposed to multiple width barcodes which is perhaps best exemplified by the UPC (Universal Product Code) barcode system.
Additionally, in order to process as many wafers or other forms of substrates through a marking system as quickly as possible, it is desirable to be able to operate in a continuous mode. However, certain materials require higher laser powers or longer laser exposure times to initiate the writing process but which nonetheless can sustain the writing process at a lower power or shorter exposure. For example, various colors of plastics have substantially different requirements for starting laser power depending upon the material's reflectivity. However, once marking starts at a given position the reflectivity decreases and/or the absorption increases so that the writing can be continued more uniformly at a lower power using a continuous wave (CW) beam or by using overlapping spots in a pulsed mode (quasi-continuous) operation. This reflectivity decrease or absorption increase is due to laser processing of the material. In the case of metals, molten metal typically has a lower reflectivity or higher absorption during lasing operations, thus allowing continuous writing at a lower power once melting has been initiated at a high power. For these materials, the continuous nature of the laser marking process is advantageous.
Thus, it is seen that if one wishes to mark a substrate, particularly a semiconductor wafer, with a single width barcode it has been the practice to turn the laser or other marking device on and off repetitively between the marking operations for each individual bar. Naturally, it is the spacing between bars of a single width code which carries the information content of the barcode itself. However, as is seen above, it is actually undesirable to turn the laser writing operation on and off since marking initiation is difficult and can adversely affect the marks that are written which become much more variable than necessary because of the fact that the laser beam is turned on and off for relatively long periods of time. A whole new initiation sequence and marking action must be initiated. This not only slows down the marking process but results in inferior marks being placed on the substrate. Naturally, such marks are harder to read.